1. Field of the Invention
The present invention relates to a trim-tilt device for a marine propulsion unit such as an outboard motor or inboard/outboard motor.
2. Description of the Related Art
Conventionally, the trim-tilt device for a marine propulsion unit has a cylinder device interposed between a hull and the propulsion unit which is tiltably supported by the hull. By controlling supply and discharge of hydraulic fluid from a hydraulic fluid supply/discharge device to a cylinder device or vice versa, the cylinder device is expanded and contracted to thereby trim and tilt the marine propulsion unit.
A prior art trim-tilt device for a marine propulsion unit is described in Japanese Patent Application No. 11-112856. This application provides a cylinder device for a trim-tilt device for a marine propulsion unit comprising a housing connected to one of a hull and a marine propulsion unit and which forms a large-diameter trim chamber. A cylinder is telescopically inserted into the trim chamber of the housing which forms a small-diameter tilt chamber. A large-diameter trim piston is fixed to an end portion of the cylinder within the trim chamber of the housing which partitions the trim chamber into a first trim chamber of a cylinder accommodation side and a second trim chamber of an anti cylinder accommodation side. A piston rod is connected to the other of the hull and the marine propulsion unit and is telescopically inserted into the tilt chamber of the cylinder. A small-diameter tilt piston is fixed to an end of the piston rod within the tilt chamber of the cylinder which partitions the tilt chamber into a first tilt chamber of a piston rod accommodation side and a second tilt chamber of an anti piston rod accommodation side.
In the above-mentioned trim-tilt device for the marine propulsion unit disclosed in Japanese Patent Application No. 11-112856, during normal forward sailing, the load of an outboard motor and an external force of a forward driving force act in the direction which contracts the cylinder device. Thus, when the cylinder device expands (up) such that a discharging hydraulic pressure of the pump acts on the second trim chamber and the second tilt chamber (a lower chamber), the pressure-receiving area of a lower chamber side of the trim piston is larger than the pressure-receiving area of a lower chamber side of the tilt piston, so that a tilt-up operation is carried out after a trim-up operation. When the cylinder device is contracted (down) such that the discharging pressure hydraulic pressure of the pump acts on the first trim chamber and the second tilt chamber (an upper chamber), the pressure-receiving area of an upper chamber side of the tilt piston is larger than the pressure-receiving area of an upper chamber side of the trim piston, which is not intercepted by the cylinder, so that the trim-down operation is carried out after a tilt-down operation.
However, the cylinder's expansion (up) involves a disadvantage, such as when a tilt-up operation occurs before a trim-up operation, when an external force, such as a backward driving force during backward sailing or an uplifting force of a wave to raise a marine propulsion unit, acts in the direction for expanding the cylinder device. In the situation that the external force Fa acts, for expanding the cylinder device, the pump is driven and a switching valve is opened. In the situation that the discharging pressure Pa of the pump acts to the second trim chamber (the lower chamber), when the force of Fa is greater than that of Pa, namely, Fa/S&lt;Pa (S: a pressure receiving area of a tilt piston), the piston rod of the tilt piston, to which Fa acts, moves ahead of the trim piston. At that time, a hydraulic pressure flow passage from the first tilt chamber to the pump absorption side is unlocked, since the switching valve is opened.
If the external force, as described above, that acts in the direction for expanding the cylinder device, such as a backward driving force during backward sailing or an uplifting force of a wave to raise a marine propulsion unit, is greater, a pressure in the pipe passage from the first tilt chamber to the pump absorption side becomes higher, the pump rotates at this pressure in the pipe passage. As the result of that, the discharge/absorption amount of the pump increases more than usual, so that an up-speed of trim/tilt becomes suddenly high.